I was able to transmit at up to around 23kHz which you may be able to make out the spike on the FFT graph on the video I made for some previous work: http://www.youtube.com/watch?v=H0DKRl8XIcU

To get up to 23kHz I used a sample rate of 48kHz.

> Is there not a low-pass filter at around 22KHz on inputs and outputs of all sound cards?

It's not a low-pass filter per se, the limit is imposed by the Nyquist–Shannon sampling theorem (http://en.wikipedia.org/wiki/Nyquist%E2%80%93Shannon_samplin...), which essentially limits the frequency spectrum to 1/2 the clock rate. Good-quality audio from a sound card is clocked at 44 KHz, which means the audio range extends only to 22 KHz.

But you can clock a modern sound card at a rate much faster than 44 KHz, so the above might be only the normal limit, not the extreme.

The built-in audio on most PCs these days supports 96kHz or even 192kHz sample rates.

The DAC may sample at that resolution, but it's another matter if the driver will pass an ultrasonic frequency. Looking at a few datasheets from various manufacturers, I don't see any mention of bandpass filters. So either they don't have them or they don't think it's worth mentioning.

No.